1. Field of the Invention
The present invention relates generally to a method of processing a silicon surface, and more particularly, to a method of etching away organic matter on a silicon surface which remains after etching away a silicon oxide film on the silicon surface.
2. Description of the Background Art
In order to fabricate a reliable semiconductor device, it is necessary to satisfactorily control an interface structure of a silicon substrate and a thin film formed on the surface of the silicon substrate. It is known that a natural oxide film is formed on a silicon surface if silicon is left in an atmosphere including oxygen. Thus, this natural oxide film must be removed before fabricating the semiconductor device.
Furthermore, the process for fabricating a semiconductor device comprises the step of forcing a silicon oxide film to be formed on a silicon surface and then, removing only a desired silicon oxide film, to form a circuit pattern.
Conventionally, the above described natural oxide film and silicon oxide film (referred to as silicon oxide film hereinafter) have been etched away using a CHF.sub.3 gas or etched away using the mixed gas of C.sub.m F.sub.n and H.sub.2. The reason is that the silicon oxide film is etched in preference to underlying silicon by using the gases. The reason why the underlying silicon is not easily etched is that CH.sub.3.sup.+ generated in plasma does not easily react with silicon so that a silicon surface is covered with organic matter of the C.sub.x F.sub.y group.
In particular, it is known that the higher the m:n ratio of C.sub.m F.sub.n becomes, the more easily the silicon oxide film is etched when the mixed gas of C.sub.x F.sub.n and H.sub.2 is used. In other words, as the m:n ratio becomes higher, the ratio of the etching rate of the silicon oxide film to the etching rate of silicon (the etching rate of the silicon oxide film/the etching rate of silicon) is improved.
However, if organic matter of the C.sub.x F.sub.y group remains on the silicon surface, some problems occur. For example, electrical resistance is increased at the time of connecting an interconnection because the organic matter is an insulating material.
Additionally, an etching damage layer is formed on the silicon surface by etching the silicon oxide film. This etching damage layer leads to the decrease in the reliability of the semiconductor device.
A method of removing the organic matter of the C.sub.x F.sub.y group and the etching damage layer on the silicon surface comprises a method of exciting a Cl.sub.2 gas by ultraviolet rays to form a Cl radical and removing the organic matter and the etching damage layer by the Cl radical. The method is described in, for example, pages 25 to 29 of the documents distributed in the 7th Symposium on Dry Process which is sponsored by Institute of Electrical Engineers in October, 1985, entitled as "Si Surface Treatment Using Deep UV Irradiation".
However, considering a case in which the organic matter of the C.sub.x F.sub.y group and the etching damage layer are removed and then, an aluminum interconnection is provided on the silicon surface, if the Cl.sub.2 gas remains on the silicon surface provided with the Al interconnection in extremely small quantities, aluminum is corroded. Thus, etching using the Cl.sub.2 gas may lead to the decrease in the reliability of the aluminum interconnection.
Furthermore, the method of removing the organic matter of the C.sub.x F.sub.y group and the etching damage layer on the silicon surface comprises a method using O.sub.2 plasma and wet processing. The method is described in, for example, an article entitled "The removal method of carbon contamination by R.I.E." in Extended Abstracts (The 48th Autumn Meeting, 1987) of The Japan Society of Applied Physics, No. 2, 19a-M-7, pp. 562.
However, by using the O.sub.2 plasma, a silicon oxide film is formed on the silicon surface after removing the organic matter of the C.sub.x F.sub.y group and the etching damage layer. On the other hand, by using the wet processing, the etching rate is increased, so that it is difficult to control etching.
Additionally, there is a method of removing a silicon oxide film from a silicon surface without leaving organic matter on the silicon surface. The method is a method of etching using a NF.sub.3 gas. For example, such a method is described in U.S. Pat. No. 4,711,698.
In this method, N which is not used for etching becomes N.sub.2 so that a residue is not left on the silicon surface.
However, by using the NF.sub.3 gas, the ratio of the etching rate of the silicon oxide film to the etching rate of silicon (the ratio of selection=the etching rate of the silicon oxide film/the etching rate of silicon) approaches 1. Therefore, the silicon and the silicon oxide film are etched by the same amount.
Thus, if there is imbalance in thickness of the silicon oxide film formed on the silicon surface, the following problems occur. A thin portion of the silicon oxide film is removed earlier than a thick portion of the silicon oxide film. Thus, underlying silicon is etched in the thin portion of the silicon oxide film until the thick portion of the silicon oxide film is removed. In particular, if there is an active region formed by implanting B or the like on the silicon surface in the thin portion of the silicon oxide film, even the active region is etched. Consequently, operating characteristics of the semiconductor device are adversely affected.
Conventionally, the organic matter and the etching damage layer remaining on the silicon surface have been etched using a gas such as CF.sub.4 and SF.sub.6 after removing the silicon oxide film. In this etching processing, the gas such as CF.sub.4 and SF.sub.6 is dissociated to form a F radical, and the organic matter and the etching damage layer are removed by this F radical. The organic matter is removed by reaction with the F radical. In addition, the F radical passes through the organic matter to react with silicon, thereby to form SiF.sub.4. The organic matter and the etching damage layer on the silicon surface are destroyed by SiF.sub.4 formed in the above described manner, to be removed.
Referring to the drawings, description is made of the conventional processing of etching away a silicon oxide film on a silicon substrate and further etching away organic matter and an etching damage layer remaining after the etching. FIG. 5 is a diagram showing the steps of such processing. FIGS. 6A to 6D are diagrams showing a state of a silicon substrate in respective steps.
As shown in FIG. 6A, a silicon substrate 2 having a SiO.sub.2 film 1 formed thereon is first prepared.
As shown in FIG. 6B, CF.sub.3.sup.+ formed by dissociating a CHF.sub.3 gas is then supplied to the silicon substrate 2 having the SiO.sub.2 film 1 formed thereon.
As shown in FIG. 6C, the SiO.sub.2 film 1 is removed. Alternatively, organic matter 3 of the C.sub.x F.sub.y group and an etching damage layer 4 remain.
Then, as shown in FIG. 6D, a F* (F radical) formed by dissociating a SF.sub.6 gas is supplied to the silicon substrate 2 on which the organic matter 3 of the C.sub.x F.sub.y group and the etching damage layer 4 remain, to remove the organic matter 3 of the C.sub.x F.sub.y group and the etching damage layer 4 from the silicon substrate.
However, the conventional etching using an etching gas such as CF.sub.4 and SF.sub.6 has the disadvantage in that a compound of carbon fluoride, sulfide or the like which is liberated simultaneously with formation of the F radical is deposited on a silicon surface, so that the silicon surface cannot be cleanly processed.